Noninvasive device for photoelectrically measuring the property of arterial blood

ABSTRACT

A noninvasive device for photoelectrically measuring a property of arterial blood is provided. Light that contacts living tissue with arterial blood is converted into a pair of electrical signals. The electrical signals are processed to provide information of the amplitude of the measured signals and are further processed to produce a final output signal that is substantially a square function of a ratio between the electrical signals representative of the amplitudes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a noninvasive device forphotoelectrically measuring a property of arterial blood, such as anoximeter or a densitometer for measurement of a pigment in blood.

2. Description of the Prior Art

The behavior of light in materials has been a topic of study in varioustheoretical works such as disclosed in "New Contributions to the Opticsof Intensely Light-Scattering Materials. Part 1," by Paul Kubelka,Journal of the Optical Society of America, Volume 38, No. 5, May 1948;"Optical Transmission and Reflection by Blood," by R. J. Zdrojkowski andN. R. Pisharoty, IEEE Transactions on Bio-Medical Engineering, Vol.BME-17, No. 2, April, 1970, and "Optical Diffusion in Blood," by CurtisC. Johnson, IEEE Transactions on Bio-Medical Engineering, Vol. BME-17,No. 2, April, 1970.

On the other hand, various practical devices or methods for measuringblood property have been disclosed in the patent literature such as U.S.Pat. Nos. 3,368,640, 3,998,550 and 4,086,915, and Japanese PatentPublication No. 53-26437.

In the specific field of the medical-optical art noninvasivemeasurements relating to the amount of a pigment in the blood, such ashemoglobin, hemoglobin oxide, bilirubin or an artificially injectedpigment, have generally taken the following form. The oximeter usuallycomprises means for providing a source light; means forphotoelectrically measuring the intensity of the source light aftercontact with a living tissue containing the arterial blood at a firstwavelength, at which the light absorption coefficients for hemoglobinand hemoglobin oxide are equal, and a second wavelength, at which thetwo light absorption coefficients greatly differ from each other, toproduce a pair of electric signals, respectively, the signals eachinclude an alternating-current (AC) component and a direct-current (DC)component; means for calculating information representative of theamplitude of the alternating-current component relative to thedirect-current component with respect to the first and secondwavelengths to produce a first and second calculated output,respectively; means for presenting a final output indicative of theoxygen saturation, and means for relating the final output with thefirst and second calculated outputs so that the final output is a linearfunction of a ratio between the first and second calculating outputs.

However, clinical experiences have recently reported that an oximeter ofthe above type was apt to show some aberration or error of measurementin the relatively lower oxygen saturation range although themeasurements are quite accurate in the higher oxygen saturation range.Thus there is still a need in the prior art to provide improvedelectro-optical measuring devices for medical use.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a non-invasive devicefor photoelectrically measuring a property of arterial blood with highaccuracy throughout a wide measurement range.

Another object of the present invention is to provide a novel oximetercapable of measuring the oxygen saturation with an improved accuracythroughout a wide oxygen saturation range.

According to the present invention the relation of the final output,such as the oxygen saturation or a density of the pigment in the blood,with the above-mentioned first and second calculating outputs isdetermined so that the final output is a square function of a ratiobetween the first and second calculating outputs, or a joint combinationof a plurality of linear functions which correspond to an approximationof the square function.

The objects and features of the present invention which are believed tobe novel are set forth with particularity in the appended claims. Thepresent invention, both as to its organization and manner of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description, taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a block diagram of a first embodiment of the presentinvention;

FIG. 2 represents graphic plots of output voltages of the photoelectricdevice of FIG. 1;

FIG. 3 represents a block diagram of a first type of the firstcalculation circuit in FIG. 1;

FIG. 4 represents a block diagram of a second type of the firstcalculation circuit in FIG. 1;

FIG. 5 represents a block diagram of a third type of the firstcalculation circuit in FIG. 1;

FIG. 6 represents a circuit diagram of the details of the square circuitin FIG. 1;

FIG. 7 represents a circuit diagrm of the details of the linear functioncircuit in FIG. 1;

FIG. 8 represents a block diagram of a second embodiment of the presentinvention;

FIG. 9 represents a circuit diagram showing a modification of the squarecircuit and dividing circuit in either FIG. 1 or FIG. 8;

FIG. 10 represents a block diagram of a third embodiment of the presentinvention;

FIG. 11 represents a circuit diagram of a detail of the linear functioncircuit of FIG. 10;

FIG. 12 represents a block diagram of a fourth embodiment of the presentinvention; and

FIG. 13 represents a block diagram of a fifth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe electro-optical art to make and use the present invention and setsforth the best modes contemplated by the inventor of carrying out hisinvention. Various modifications, however, will remain apparent to thoseskilled in the art, since the generic principles of the presentinvention have been defined herein specifically to provide a noninvasivedevice for photoelectrically measuring a property of blood.

FIG. 1 represents a block diagram of a first embodiment of the presentinvention in the form of an oximeter, in which a source light having awide wavelength band emerges from a light source 1 to enter interferencefilters 3 and 4, respectively, by way of transmission through the livingtissue 2 being monitored. The peak of transmission of interferencefilter 3 is at a wavelength, λ₁ at which the light absorptioncoefficient for the hemoglobin is equal to that for the hemoglobinoxide, while the peak of transmission of intereference filter 4 is at awavelength, λ₂ wherein the two light absorption coefficients are greatlydifferent from each other. Thus, photoelectric devices 5 and 6 areresponsive to the intensities of light at wavlengths, λ₁ and λ₂,respectively. The changes depending on the lapse of time in outputvoltages of photoelectric devices 5 and 6 are shown in FIG. 2, whereinan alternating-current (AC) component is added to a direct-current (DC)component since the light transmitted through living tissue is generallyabsorbed by muscle, bone, venous blood and arterial blood, and thequantity of arterial blood periodically changes in response to thepulsation of the heart in contrast to other unchanged factors.

A pair of first calculation circuits 7 and 8 each calculate informationrepresentative of the amplitude of the alternating-current componentrelative to the direct-current component which provides a referencelevel with respect to wavelengths, λ₁ and λ₂, respectively. The pair ofoutputs, which are derived from the first calculation circuits 7 and 8,are squared by a pair of square circuits 9 and 10, respectively.Dividing circuit 11 is for obtaining the ratio between the outputs fromthe pair of square circuits 9 and 10. The output, X of dividing circuit11, which corresponds to the square of the ratio between the outputs ofthe pair of first calculation circuits 7 and 8, is transmitted to alinear function circuit 12 to be multiplied by a first constant, A₁ andadded to a second constant, B₁. The results of the calculation by thelinear function circuit 12 is representative of the oxygen saturation ofblood and can be displayed by some indicator 50 such as a meter or adigital display circuit.

In more detail, according to the present invention, the output voltages,E₁ and E₂ of the pair of photoelectric devices 5 and 6 are defined asfollows, respectively: ##EQU1## wherein: K₁ and K₂ represent a pair ofconstants determined by photosensitive elements in the photoelectricdevices 5 and 6, respectively; I₀₁ and I₀₂ represent the intensities ofsource light at wavelengths, λ₁ and λ₂, respectively; F_(T1) and F_(T2)represent the light absorption coefficients of materials other than thearterial blood at wavelengths, λ₁ and λ₂, respectively; D₁ and D₂represents a pair of constants depending on the scattering coefficientand the absorption coefficient of the arterial blood at wavelengths, λ₁and λ₂, respectively; g₁ and g₂ represent a pair of constants dependingon the scattering coefficient of the arterial blood at wavelengths λ₁and λ₂ and the total density of the hemoglobin and the hemoglobin oxide,respectively; β₁ and β₂ represent the light absorption coefficients ofthe arterial blood at wavelengths, λ₁ and λ₂ ; d represents an averagethickness of the arterial blood; and Δd represents the change dependingon the lapse of time in the thickness of the arterial blood.

In determining the above equations (1) and (2), the present inventionregards the light measured by way of transmission through the livingtissue as determined under the conditions that:

(i) the influence of scattering by the arterial blood is not negligible;

(ii) the optical path in the living tissue is sufficiently long; and

(iii) the scattering coefficient is sufficiently great relative to theabsorption coefficient.

The pair of first calculation circuits 7 and 8 each may be practicallydesigned in accordance with any one of FIGS. 3, 4 and 5. Specifically,the circuit in FIG. 3 comprises a first logarithmic conversion circuit13 to obtain a logarithm of the output from the photoelectric device 5or 6, a low-pass filter 14, a second logarithmic conversion circuit 15to obtain a logarithm of the direct-current component of the output fromthe photoelectric device 5 or 6, and a differential amplifier 16 tosubtract the output of circuit 15 from the output of circuit 13, forcalculating a logarithm of a ratio of the whole output of photoelectricdevice 5 or 6 to the direct-current component thereof. On the otherhand, the circuit in FIG. 4 comprises a high-pass filter 17 to obtainthe alternating-current component of the output from photoelectricdevice 5 or 6, and a dividing circuit 18, for calculating a ratio of thealternating-current component of the output of photoelectric device 5 or6 to the whole output thereof. Further, the circuit in FIG. 5 comprisesa logarithmic conversion circuit 19 to obtain the logarithm of theoutput of photoelectric device 5 or 6 and a high-pass filter 20 toobtain the alternating-current component of the output of circuit 19.

In designing the oximeter, the pair of first calculating circuits 7 and8 should adopt the same type of circuit, although the type may beselectable among FIGS. 3 to 5. Any one of the circuits in FIGS. 3 to 5is substantially capable of calculating information representative ofthe relative amplitude of the alternating-current component of theoutput of photoelectric device 5 or 6, although the degree ofapproximation is individually different.

Thus, the output voltages, E₃ and E₄ of the pair of first calculatingcircuits 7 and 8 in FIG. 1 are given in accordance with the followingequations, respectively: ##EQU2##

FIG. 6 represents an example of a detailed circuit applicable for squarecircuits 9 and 10, and its function is self-evident to a person skilledin the optical electrical art without any further explanation. However,it is pointed out that block 21 represents a rectifier and smoothingcircuit connected to the output of the first calculation circuit 7 or 8.

The output voltages, E₅ and E₆ of the pair of square circuits 9 and 10in FIG. 1 is as follows:

    E.sub.5 =(g.sub.1 Δd).sup.2 β.sub.1             (5)

    E.sub.6 =(g.sub.2 Δd).sup.2 β.sub.2             (6)

Further, the output voltage, E₇ of the dividing circuit 11 is asfollows: ##EQU3##

Here, it should be noted that the light absorption coefficients, β₁ andβ₂ are generally defined by the following equations:

    β.sub.1 =C{S[a.sub.1 (HbO.sub.2)-a.sub.1 (Hb)]+a.sub.1 (Hb)}(8)

    β.sub.2 =C{S[a.sub.2 (HbO.sub.2)-a.sub.2 (Hb)]+a.sub.2 (Hb)}(9)

wherein: a₁ (HbO₂) and a₂ (HbO₂) represent the light absorptioncoefficients of hemoglobin oxide, HbO₂ at wavelengths, λ₁ and λ₂,respectively; a₁ (Hb) and a₂ (Hb) represent the light absorptioncoefficients of hemoglobin, Hb at wavelengths, λ₁ and λ₂ ; C representsthe total density of the hemoglobin and the hemoglobin oxide in thearterial blood; and S represents the oxygen saturation in the arterialblood.

Equation (8) is simplified as follows since a₁ (HbO₂)=a₁ (Hb) atwavelength λ₁ :

    β.sub.1 =C a.sub.1 (Hb)                               (10)

From equations (7), (9) and (10), the following equation (11) results:##EQU4## Therefore, ##EQU5##

The constants A₁ and B₁ are defined by the following equations: ##EQU6##

The above equation (12) can be further summarized as follows: ##EQU7##

This means that the oxygen saturation S is calculated as a linearfunction of (E₄ /E₃)², which represents a square of the ratio betweenthe outputs of the pair of first calculation circuits 7 and 8. Linearfunction circuit 12 performs the calculation in accordance with thelinear function defined by equation (12). FIG. 7 represents an exampleof a detailed circuit applicable to a linear function circuit and itsfunction will be self-evident with no additional explanation. However,it should be noted that the circuit constants R₁ to R₃ and V₃ have tofulfill the following equations: ##EQU8## The values for A₁ and B₁ canbe determined in accordance with equations (13) and (14), respectively.

FIG. 8 represents a second embodiment of the present invention, in whichthe same elements as those in FIG. 1 are indicated by the same symbolsand explanations thereof are accordingly omitted.

In summary, the same embodiment in FIG. 8 is substantially identicalwith the first embodiment in FIG. 1 except that the outputs of the pairof first calculation circuits 7 and 8 are subjected to division individing circuit 22 and, in turn, squared by a single square circuit 23,in place of each being squared by a pair of square circuits 9 and 10prior to being divided by dividing circuit 11 in FIG. 1. The circuit inFIG. 6 is also applicable as square circuit 23 in FIG. 8. The outputvoltage, E₈ of dividing circuit 22 is given as follows: ##EQU9##Further, the output voltage, E₉ of square circuit 23 is given asfollows: ##EQU10## which is identical with equation (7).

FIG. 9 provides a modification of the embodiments in FIGS. 1 and 8.Namely, the circuits 9, 10 and 11 in FIG. 1 or the equivalent circuits22 and 23 in FIG. 8 can be alternatively constructed as a compositecircuit in FIG. 9 capable of both the squaring and dividing functions,and its operation will be self-evident without additional explanationexcept that the rectifier and smoothing circuits 24 and 25 are connectedto the pair of first calculation circuits 7 and 8, respectively, andthat the gain of the differential amplifier OP in FIG. 9 should be setat twice the value for the purpose of obtaining the following output atterminal 26: ##EQU11##

FIG. 10 represents a third embodiment of the present invention, in whichthe same elements as those in FIG. 8 are indicated by the same symbolsto avoid any redundant explanation. The third embodiment is designed inaccordance with the findings that equation (15), which is a squarefunction of E₄ /E₃, can be approximately substituted by a jointcombination of the following linear functions of E₄ /E₃, provided thatthe oxygen saturation is greater than 50 percent: ##EQU12## wherein, A₂,B₂, B₃ and M are given constants, respectively. In FIG. 10, linearfunction circuit 26 is capable of determining whether or not E₄ /E₃ isgreater than M in addition to calculating the oxygen saturation S inaccordance with equation (16) or (17) selected in response to such adetermination.

FIG. 11 represents an example of a circuit applicable to such a linearfunction circuit 26. In FIG. 11, the output, E₄ /E₃ of dividing circuit22 is transmitted to terminal 27 as a positive voltage. At terminal 28,a negative voltage V₅, which fulfills M=-V₅, is transmitted.

When E₄ /E₃ <-V₅, the output of differential amplifier OP₂ is zero toallow only the output of differential amplifier OP₁ to be transmitted todifferential amplifier OP₃. The output voltage of differential amplifierOP₁ is designed to be equal to the following value for the input, E₄ /E₃by means of adjusting the variable resistors VR₁ and VR₂ : ##EQU13##Thus, the output voltage of differential amplifier OP₃ connected toterminal 29 is as follows when E₄ /E₃ <-V₅ : ##EQU14## which isidentical with equation (16).

On the other hand, when E₄ /E₃ ≧-V₅, the output voltage or differentialamplifier OP₂ is as follows, provided that R₅ represents the resistanceof variable resistor VR₂ : ##EQU15## The output voltage at terminal 29in this case is as follows since the voltages of above values (18) and(20) are both transmitted to differential amplifier OP₃ : ##EQU16##which is identical with equation (17) provided that: ##EQU17##

Since the third embodiment is only an example of approximatelysubstituting the square function (15) by a combination of a plurality oflinear functions, it is needless to say that any other approximation bymeans of utilizing another combination of a plurality of linearfunctions, e.g., three or more linear functions, can be possible withinthe scope of the present invention.

FIG. 12 represents a fourth embodiment of the present invention, inwhich the analog outputs E₃ and E₄ of the pair of first calculationcircuits 7 and 8 are converted into digital signals P₁ and P₂ by meansof a pair of rectifiers and smoothing circuits 30 and 31 and A-Dconverters 32 and 33, respectively. The digital signals P₁ and P₂ can beprocessed by microprocessor 34 with random access memory (RAM) 35 andread only memory (ROM) 36 to indicate the oxygen saturation by means ofa digital display 37.

For example, microprocessor 34 is programmed to carry out the followingcalculation, as in the first and second embodiments: ##EQU18##

Or, alternatively, microprocessor 34 is programmed to discriminatebetween the following cases (i) and (ii) to select one of them, andcarry out the calculation in accordance with the selected case, as inthe third embodiment: ##EQU19##

The above constants A₁, B₁, A₂, B₂, A₃ and M may be stored in RAM 35 orROM 36, or alternatively be inputted by means of a combination of aplurality of switches representative of a digital code.

Another example of the function of microprocessor 34, RAM 35 and ROM 36in the fourth embodiment in FIG. 12 is as follows. Namely, variousoxygen saturation values, S₀, S₁, S₂, . . . , S_(i), . . . , S_(n-1) andS_(n) have been previously calculated in accordance with equation (21)for various possible values (P₂ /P₁)₀, (P₂ P₁)₁, (P₂ P₁)₂, . . . , (P₂P₁)_(i), . . . , (P₂ /P₁)_(n-1) and (P₂ /P₁)_(n) and stored in ROM 36 ataddresses K, K+1, K+2, . . . , K+i, . . . K+n, respectively. And (P₂/P₁)_(i) fulfilling the following condition is searched with respect tothe actually obtained ratio P₂ /P₁ :

    (P.sub.2 /P.sub.1).sub.i ≦P.sub.2 /P.sub.1 <P.sub.2 /P.sub.1).sub.i+1

to determine address K+i at which the desired oxygen saturation S_(i) isstored. The oxygen saturation S_(i) is read out from ROM 36 anddisplayed at digital display 37.

Strictly speaking, such an oxygen saturation S_(i) is not accuratelyequal to the oxygen saturation S which would be directly calculated inaccordance with equation (21). However, S_(i) is practically regarded asS if the number, n is sufficiently great.

Or, if n is desired to be not so great, than a further modification ispossible such that S is calculated by microprocessor 34 in accordancewith a suitable interpolation such as: ##EQU20##

Although FIG. 12 discloses that the device includes two A-D converters32 and 33, such a modification is possible that only one A-D converteris alternatively utilized which is connected to both circuits 30 and 31by way of a suitable multiplexer controlled through microprocessor 34.

FIG. 13 represents a fifth embodiment of the present inventionconstructed as a densitometer for a desired pigment, such as bilirubinor an artificially injected pigment, in the arterial blood in contrastto the foregoing embodiments which are constructed as an oximeter. InFIG. 13, the same elements as those in FIG. 1 are represented by thesame symbols without additional explanation. Since the fifth embodimentis a densitometer, the peak of transmission of interference filter 38 isat wavelength, λ₃ at which the light absorption by the pigment does notoccur, while the peak of transmission of interference filter 39 is at awavelength, λ₄ at which the light absorption by the pigment effectivelyoccurs. Therefore, the output voltages E₁₀ and E₁₁ of the pair ofphotoelectric devices 5 and 6 are as follows, respectively, which aresubstantially similar to equations (1) and (2): ##EQU21## Wherein: I₀₃and I₀₄ represent the intensities of source light at wavelengths λ₃ andλ₄, respectively; F_(T3) and F_(T4) represent the light absorptioncoefficients of materials other than the arterial blood at wavelengthsλ₃ and λ₄, respectively; D₃ and D₄ represents a pair of constantsdepending on the scattering coefficient and the absorption coefficientof the arterial blood at wavelengths λ₃ and λ₄, respectively; g₃ and g₄represent a pair of constants depending on the scattering coefficientsof the arterial blood at wavelengths λ₃ and λ₄ and the total density ofthe hemoglobin and the hemoglobin oxide, respectively; and β₃ and β₄represent the light absorption coefficient of the arterial blood atwavelengths λ₃ and λ₄.

The output voltages, E₁₂ and E₁₃ of the pair of first calculationcircuits 7 and 8 are as follows as in the first embodiment: ##EQU22##Further, the output voltage, E₁₄ of dividing circuit 11 is as follows:##EQU23## Here, it should be noted that the light absorptioncoefficients, β₃ and β₄ are generally defined by the followingequations, provided that μ₁ and μ₂ represent the light absorptioncoefficients of the total of hemoglobin and hemoglobin oxide atwavelengths λ₃ and λ₄, respectively, C' represents the density of thepigment in the blood and μ₂ ' represent the light absorption coefficientof the pigment at wavelength λ₄ :

    β.sub.3 =Cμ.sub.1                                  (26)

    β.sub.4 =Cμ.sub.2 +C'μ'.sub.2                   (27)

From equations (25) to (27), the following equation results: ##EQU24##Therefore, ##EQU25##

The constants A₄ and B₃ in circuit 40 of FIG. 13 are defined as follows:##EQU26## The equation (29) can be summarized as follows: ##EQU27##

Circuit 40 in FIG. 13 carries out a calculation in accordance with thelinear function (32) with respect to (E₁₃ /E₁₂)², which is the output ofdividing circuit 11. As the value for C in equations (30) and (31), anaverage value of various healthy men is applicable, or alternatively apersonal value may be previously measured and applied, although thevalue for C does not particularly depend on the individual persons, butis rather common. Thus, the fifth embodiment in FIG. 13 functions as adensitometer for a desired pigment in the blood.

Clinical experiments in utilizing the device according to the presentinvention have shown that the measurement result is quite accuratethroughout a wide range of variation in the oxygen saturation or thedensity of desired pigment in the blood in comparison with the prior artnoninvasive oximeter using a pair of wavelengths.

While the above embodiments have been disclosed as the best modespresently contemplated by the inventor, it should be realized that theseexamples should not be interpreted as limiting, because artisans skilledin the field, once given the present teachings can vary from thesespecific embodiments. Accordingly, the scope of the present inventionshould be determined solely from the following claims.

What is claimed is:
 1. A noninvasive device for photoelectricallymeasuring a property of arterial blood in living tissue comprising:meansfor providing a source light to enter into living tissue, the lightmeans remains outside the living tissue and only the light passes intothe living tissue; means for photoelectrically measuring the intensityof the source light emerging from the living tissue after contact withthe arterial blood therein, at a pair of separate wavelengths, and forproducing a pair of electric signals, respectively, each signalincluding an alternating-current component and a direct-currentcomponent; means for calculating information from the signalsrepresentative of the relative amplitude of the alternating-currentcomponent compared to the direct-current component for each of themeasured wavelengths and for producing first and second calculatedoutputs, respectively representing the relative amplitudes at eachwavelength; means for obtaining a final output signal that is a linearfunction of the square of a ratio between the first and secondcalculated outputs; and means for indicating the property of thearterial blood in response to the final output.
 2. The invention ofclaim 1, wherein said obtaining means includes means for approximatingthe linear function of the square of the ratio as a joint combination ofa plurality of linear functions of the ratio.
 3. The invention of claim2, wherein said approximating means includes means for selecting one ofthe plurality of linear functions for use in approximating, the oneselected being dependent upon the level of the ratio.
 4. The inventionof claim 1, wherein the calculating means includes a pair of means,responsive to the pair of electric signals of the photoelectricallymeasuring means, for producing the first and second calculated outputs,respectively.
 5. The invention of claim 1, wherein the pair ofwavelengths consist of a first wavelength at which the light absorptioncoefficient of the hemoglobin and that or the hemoglobin oxide are equalto each other, and a second wavelength at which the two light absorptioncoefficients are different from each other, and wherein the final outputsignal is representative of the oxygen saturation.
 6. The invention ofclaim 1, wherein the pair of wavelengths consists of a first wavelengthat which light absorption by a pigment to be measured in the blood doesnot occur, and a second wavelength at which absorption by the pigmenteffectively occurs relative to the first wavelength, and wherein thefinal output signal is representative of the density of the pigment. 7.The invention of claim 1, wherein the obtaining means includes amicroprocessor and an associated digital memory circuit.
 8. Theinvention of claim 1, wherein the obtaining means includes means forpreviously storing various values for the final output signal inrelation to a variety of possible combinations of values for the firstand second calculated outputs, and means for searching a specific valuefor the final output signal in accordance with a specific combination ofvalues for the first and second calculated outputs actually obtained bythe calculating means.
 9. A noninvasive device for photoelectricallymeasuring a property of arterial blood in living tissue comprising:meansfor providing a source light to enter into living tissue from theoutside thereof; means for photoelectrically measuring the intensity ofthe source light emerging from the living tissue after contact with thearterial blood contained therein, at a predetermined pair of wavelengthswithin the source light and for producing a pair of electric signals,respectively representing the intensity at each wavelength; means forpresenting information representative of the relative amplitude of eachof the electric signa with respect to a reference level; means forobtaining a final output signal that is a linear function of the squareof a ratio between the information from said presenting means for thefirst of the pair of wavelengths and that for the second thereof; andmeans for indicating the property of the arterial blood in response tothe final output signal.
 10. A noninvasive device for photoelectricallymeasuring the oxygen saturation of arterial blood in living tissuehaving hemoglobin and hemoglobin oxide comprising:means for providing asource light to enter into living tissue, the light means remainsoutside the living tissue and only the light passes into the livingtissue; means for photoelectrically measuring the intensity of thesource light emerging from the living tissue after contact with thearterial blood therein, at a pair of separate wavelengths, and forproducing a pair of electric signals, respectively, each signal includesan alternating-current component and a direct-current component, whereinthe pair of wavelengths consists of a first wavelength at which thelight absorption coefficient of the hemoglobin and that of thehemoglobin oxide are equal to each other, and a second wavelength atwhich the two light absorption coefficients are different from eachother; means for determining the amplitude of the alternating-currentcomponent of each of the signals, respectively and for producing firstand second outputs, respectively representing the relative amplitudes ofthe alternating current component of each signal with respect to theD.C. component; means for obtaining a final output signal that is alinear function of the square of a ratio between the first and secondoutputs including a microprocessor and a digital memory circuit, thedigital memory circuit having stored therein various values for thefinal output signal in relation to a variety of possible respectivecombinations of values for the first and second outputs, themicroprocessor searching a specific value for the final output signal inaccordance with a predetermined combination of values for the first andsecond outputs actually obtained by the determining means; and means forindicating the oxygen saturation of the arterial blood in response tothe final output signal value selected.